1. Field of the Invention
The present invention relates to an image capturing apparatus that obtains images by an image sensor with RGB pixels receiving incident light from an object, and an image processing method.
2. Description of the Related Art
Generally, object light has spectral characteristics of a light source or spectral characteristics as a combination of the light source and the object reflectance. Whether for film cameras that use photosensitive film or for digital cameras that obtain digital image data, technologies for determining light-source information based on the spectral characteristics of the light source in order to accurately reproduce the color of the object have been extensively developed.
For example, a technology is disclosed in which a camera is provided with a colorimetric sensor for measuring the environment where the camera is present and the light-source information determined by that sensor is recorded on a recording medium (for example, see Japanese Patent Laid-Open No. 05-197009). Furthermore, a technology is disclosed in which an object is recorded as an image of spectral distribution or spectral reflectance by using a multi-band camera that captures an image through a plurality of types of filters having different transmission wavelength ranges (for example, see Japanese Patent Laid-Open No. 2007-278950).
On the other hand, with respect to image capturing apparatuses such as digital cameras, chromatic aberration of their objective lenses is a factor that causes a deterioration of the image quality. In order to correct this chromatic aberration by image processing, the following technologies are known. For example, a technology is disclosed that performs chromatic-aberration correction with use of the dependence of chromatic aberration on the image height from the center of an imaging device, which is an imaging plane of that lens, and the wavelength of incident light (for example, see Japanese Patent Laid-Open No. 06-292207). Moreover, a technology that uses a spacial filter having inverse characteristics of a point spread function (PSF), or OTF characteristics from which inverse characteristics PSF are generated, to perform chromatic-aberration correction is disclosed (for example, see Japanese Patent Laid-Open Nos. 62-12976 and 04-88765).
More exactly, chromatic-aberration correction as described above needs to be performed according to the spectral characteristics of light rays incident on an image sensor. Thus, a technology for controlling chromatic-aberration correction based on color temperature or white balance information in a camera is disclosed (for example, see Japanese Patent Laid-Open No. 2007-158628).
Generally, light incident on an image sensor depends on the spectral characteristics of a light source, the spectral reflectance of an object, and furthermore, the emission spectral characteristics of an object that emits light. At this time, there is a problem that if a feature quantity of light rays incident on the image sensor is determined based on the respective pixel values of the R, G, and B bands, the color temperature, the colorimetric value, or the white balance information, accuracy cannot be ensured. For example, when it is desired to distinguish between an artificial green object and a plant and separately perform optimum color processing for each of them, attention needs to be paid to the difference in spectral characteristics between them. However, in such a case, even when RGB pixel values of the artificial green object and the plant are obtained, discrimination is impossible if their colorimetric values are close to each other, and thus, the intended processing cannot be performed.
On the other hand, it is possible to estimate the spectral characteristics of light incident on the image sensor by using a multi-band camera. For example, a method in which, in a camera having a monochrome image sensor, a plurality of images are captured while switching filters of respective bands or a method that uses a camera in which all the bands are disposed on a single-plate image sensor are conceivable. However, the former method has the problems that capturing an image is extremely complicated and furthermore an image of a moving object cannot be captured. The latter method has the problem that the resolution is lower than that of RGB three-band cameras with the same number of pixels.
Moreover, in the case of realizing correction of the chromatic aberration as described above by image processing, it is necessary to discriminate between incident light rays having high power peaks at wavelengths between adjacent bands (RG and BG) and incident light rays having average power in each of the R, G, and B bands. If it is not possible to determine the difference in spectral characteristics between these incidents light rays, chromatic aberration not only cannot be properly corrected for, but also may rather become worse.